With the spread of the Internet, the amount of data to be communicated has increased. In order to cope with this, it is necessary to increase the capacity of a transmission path. As one of the techniques for realizing large capacity, a multi-level modulation system (Quadrature Amplitude Modulation: QAM) is known. An optical signal modulated by the QAM system with a transmitter is demodulated with a digital coherent optical receiver.
In QAM optical communication, a nonlinear effect applied to the optical signal when propagating through the transmission path becomes a major problem. When the optical signal is applied with the nonlinear effect in the transmission path, the phase of the optical signal rotates. Since the QAM system handles complicated phase information, if the optical signal is applied with phase rotation due to the nonlinear effect, correct phase information cannot be demodulated at the time of reception.
Non-Patent Document 1 describes a nonlinear compensation system which is called Back Propagation. This compensation system is a system in which dispersion compensation is performed little by little and nonlinear compensation is performed immediately after respective dispersion compensation to compensate for waveform distortion while going back to a propagation waveform from the reception side to the transmission side.
However, in Back Propagation, when the dispersion compensation function and the nonlinear compensation function are combined as one nonlinear compensation stage, it is necessary to increase the number of nonlinear compensation stages. The dispersion compensation function is realized by a linear distortion compensation circuit, and the nonlinear compensation function is realized by a nonlinear distortion compensation circuit. The linear distortion compensation circuit includes an FFT/IFFT circuit in order to perform dispersion compensation in a frequency domain. Since the FFT/IFFT circuit is large in circuit scale, considering the mounting area of LSI or power consumption, only a few FFT/IFFT circuits can be mounted on one signal processing device.
Non-Patent Document 1 describes a compensation system which is called Filtered Back Propagation. In Filtered Back Propagation, the time average of the amount of phase rotation calculated from the signal strength is used for nonlinear compensation, thereby reducing the number of nonlinear compensation stages.
As a system which further reduces the number of nonlinear compensation stages, Perturbation Back Propagation is known. In Perturbation Back Propagation, in addition to cancelling phase rotation due to the signal strength, phase rotation due to the perturbative component is cancelled, whereby it is possible to reduce the nonlinear compensation stages, for example, to three stages (Non-Patent Document 2).